The purpose of the Cosmic-Ray Energy Deposition Experiment (CEDEX) is to characterise the space radiation environment as encountered by PHASE-3D over both short-term and long-term time-scales. The experiment occupies a single module and consists of two sub-systems: the Total Dose Experiment (TDE) and the Cosmic Particle Experiment (CPE).
CEDEX is a re-housed version of the Cosmic-Ray Experiment (CRE) which already flies on the KITSAT-1 (KITSAT-OSCAR-23) and PoSAT-1 micro-satellites in low-Earth orbit. Inter-comparison of the results of all these missions should bring new knowledge on the dynamics of the Earth's radiation belts.
Each RADFET sensor consists of a matched pair of p-channel MOSFETs, one of which is biased during exposure (MEASURE MODE), whilst the other remains unbiased. In READ MODE, a constant current (6 microamps) is switched to each RADFET in turn, and the threshold voltage is measured. This voltage is a function of temperature as well as dose, but the temperature effect can be largely compensated for by noting the difference in threshold voltage change between the biased and un-biased RADFETs on a particular sensor. The gradual shift in this voltage difference is approximately proportional to the accumulated ionising radiation dose.
To maintain the correct biasing conditions, the TDE part of the CEDEX experiment be powered up as soon as possible after launch, and be kept powered-up during the entire mission.
During READ MODE, the data from the RADFETs and associated temperature and current sensors are collected by the internal CAN-controller, which formats the data and passes them on for storage in the RUDAK module via the spacecraft's CAN bus.
The sub-multiplexed output of the is as follows:
Channel Number Output 0 Switch 0 Constant Current Source 1-6 Switch 1-6 Not Used in CEDEX Version 7 Switch 7 RADFET 3 (biased) CEDEX 1 8 Switch 8 RADFET 3 (un-biased) CEDEX 1 9 Switch 9 RADFET 4 (biased) CEDEX 2 10 Switch 10 RADFET 4 (un-biased) CEDEX 2 11 Switch 11 RADFET 5 (biased) CEDEX 3 12 Switch 12 RADFET 5 (un-biased) CEDEX 3 13 Switch 13 RADFET 6 (biased) CEDEX 4 14 Switch 14 RADFET 6 (un-biased) CEDEX 4 15 Switch 15 Constant Current Source 16 Temperature Sensor 0 CEDEX 1 17 Temperature Sensor 1 CEDEX 2 18-21 Temperature Sensor 2-5 Not Used in CEDEX Version 22 RADFET Offset Voltage (approx. 4.5V) 23 Temperature Sensor Offset Voltage (approx. 1.4V) 24-31 = repeat of channels 16-23.
PHASE-3D represents a unique opportunity to characterise this high-inclination high-eccentricity orbit, which is subject to significant proton fluxes from the inner Van Allen belt over the South Atlantic. These data are also be of great use in evaluating the radiation performance of the electronics used in the PHASE-3D satellite.
The CPE consists of a single 30mm x 30mm PIN diode detector (900 mm2 active area), 300 microns in depth, housed in a separate screened aluminium box within the CEDEX module box. This is connected to a charge amplifier and a pulse-shaping circuit which in turn is connected to an event-driven, hardware-logic controlled pulse-height multi-channel analyser. The experiment is controlled autonomously by a 87C51 microcontroller with its own data-storage RAM and built-in data-compression and communications software. This sends data to an internal CAN-controller which formats and sends them on to the RUDAK module via the spacecraft's CAN bus.
Charged particles passing through the detector deposit energy within the silicon lattice primarily via coulomb interactions with the bound electrons of the silicon atoms. This energy appears as the formation of charge-carriers, and for silicon, one electron-hole pair is produced for every 3.6 eV of energy deposited. Thus, the charge induced in the detector is proportional to the energy deposited within the detector. This charge appears very promptly (governed by the transit-time of the particle - usually a few picoseconds), and can be swept-out of the detector at a rate which depends upon the mobility of the carriers and the response-time of the detector electronics (10s-100s of nanoseconds). This puts a physical upper-limit on the rate at which particles can be detected and counted.
The detector is chosen to be able to cope with both low and high flux conditions, and has a "dead-time" of approximately 5 microseconds (determined by the hardware used). This represents a maximum practical rate for a low-power, high resolution system.
The multi-channel analyser has 512 channels, each of equal width equivalent to approximately 0.046 pC of charge deposited in the detector (i.e. an equivalent normal-incidence LET of 14.4 MeV cm2 g-1)). The total charge-range of the instrument is approximately 0.185 pC to 23.7 pC, equivalent to a normal-incidence particle LET range of 57.7 MeV cm2 g-1) to 7370 MeV cm2 g-1). In operation, the charge-pulses from the PIN diode are amplified and shaped, and the pulse-height is recorded by a fast semi-flash 10-bit analogue-to-digital converter (ADC). The output of the ADC is used to address self-incrementing memory locations which can hold a count of up to 224 (16,777,215 counts). These counts are integrated over a fixed period of 600 seconds - internally sub-divided into four periods of 150 seconds to improve orbit-position/time resolution. Thus, 8K-bytes of data are recorded for each 600 second integration period, and are transmitted using a loss-free data-compression coding scheme to RUDAK via the CAN-bus interface. RUDAK will store and time-stamp the data, which will be downloaded each day.
For simplicity of design, the three bytes of count-data per channel are stored in four bytes of data memory. The fourth byte is used to store a fixed bit- pattern for examination of any possible single-event upsets (SEUs).
The aluminium shielding around the detector should limit the count-rates to around 10,000 counts per second in the most sensitive channel when passing through the heart of the South Atlantic Anomaly (SAA). The detector is designed to count particles up to a uniform rate of 200,000 particles per second (i.e. particles which arrive within 5 microseconds of each other cannot be distinguished). This should give the detector a counting efficiency in excess of 95%.
In operation the CEDEX module produces packets containing CPE data every 10 minutes. These packets are processed by the internal CAN-controller, which also samples the 32 sub-multiplexed channels from the TDE.
RUDAK handles time-stamping and filing of CPE and TDE data, and data-files representing each day's activity are stored on-board until they are requested by the ground (usually each working day). Files stored in the RUDAK are protected against single-event upset (SEU) by software coding/wash routines, and the downlink is error-controlled by the packet communications protocols ensuring error-free data at the ground-station.
The CPE should be operated as often as possible in order that unusual events (such as solar-flares) may be recorded. However, as the power consumption is relatively high (approximately 2.5 W), it may not always be possible to operate it under all illumination conditions.